[RWilson]

That's a great link, I had not seen that article before. Though, in practice,the actual framerate to vary from the specified by much more than the -10 / +5 fps that he lists.

His final sentences might be relevant to this topic:

On high CPU load we might actually cut [the max framerate] into half, e.g. 30 frames/sec. OS X already does this in certain conditions.

Wish he had expanded on that a little more. Any idea what conditions cause OS X to halve the framerate?